The backlight module is an important component of a liquid crystal display (LCD) device. In various types of the backlight modules, edge-lighting backlight module is used widely. As shown in FIG. 1, an edge-lighting backlight module comprises a light guide plate 1 and a plurality of light sources (e.g., LED) 2. The light sources are arranged outside of one side surface of the light guide plate 1 with intervals between the light sources. The optical axis of each of the light sources is perpendicular to said side surface. Light 9 is emitted from the light source 2 into the light guide plate 1, and subjected to series of reflections and refractions, and then emerges from the light emitting surface (upper surface of the light guide plate) so as to be utilized by the LCD device.
Since the light sources are arranged with intervals therebetween, and the refraction angle is smaller than the incident angle when the light 9 is emitted from the light sources into the light guide plate 1 for the reason that the light guide plate 1 is an optically denser medium relative to the air, the intensity between the light 9 emitted into the light guide plate from the positions opposite to the light sources 2 and the positions opposite to the intervals is different. Consequently, dark areas 8 occur between the light sources 2. If the mixed light is insufficient, alternately dark and bright areas may be produced at the edge of the LCD device, which is known as “firefly phenomenon” (also called “hot spot badnes”). Particularly, as the development of the technology, the size of the light guide plate 1 is getting larger and larger, and the number of the light sources 2 is getting less and less, and thereby the firefly phenomenon is more and more apparent.
In order to eliminate the firefly phenomenon, various solutions have been proposed, such as changing the distribution of the scattering points in the light guide plate, adding light mixing microstructures (such as microprisms or recesses) at the light-incident positions for the light sources, and so on. However, these existing solutions will lead to disadvantages of complex structure, difficult manufacture process and high cost of the product. Furthermore, when the number of the light sources is reduced to a certain extent, these solutions cannot eliminate the firefly phenomenon any more.